The present invention relates to injection devices for injecting medicament in fluid form having high viscosity which means that these devices require high forces in order to press the fluid through a needle when injecting the medicament.
Auto-injectors, or pen-injectors have been on the market for many years. One of the first auto-injectors was developed for war-times, which was activated by pressing the injector against a body part for activating it. The main concern was to have the medicament injected as fast as possible, without much concern for the patient or for handling safety aspects. During the recent years some medicaments have been developed such that these have to be injected by the patients themselves. Therefore, depending on the intended use and type of medicament, it has also been developed injection devices having a varying degree of automatic functions to facilitate the injection of medicaments in a reliable and safe way for patients and even for trained personnel; e.g. physicians, nurses.
Auto-injector devices having an automated injection function often comprises a housing, a spirally wound compression spring acting on a plunger rod which in its turn acts on a stopper inside a medicament container for expelling the medicament through an attached needle to the container. Normally, one end of the spring is often abutting an inner end surface of the housing, which means that the housing has to be dimensioned to the force of the spring. When fluids with high viscosity are to be injected using an auto-injector, high forces are required to expel the medicament through a fine needle. Consequently, the spring becomes very large both regarding the diameter of the wound spring and also the diameter of the thread of the wire. The size of the spring means that the device becomes large, and for some applications and customers, such sizes of the devices are not acceptable.
A solution to said problem is disclosed in document EP 1 728 529 which discloses a medicament delivery device arranged with an energy accumulating member, e.g. a flat spiral spring, capable of providing an output torque to a plunger rod driving member which is adapted to engage a threaded plunger rod. The rotation of the plunger rod driving member due to the output torque of the spring rotates the plunger rod and allows said rod to be driven forwardly within a medicament container for expelling said medicament. When said device is used as an injection device; a user e.g. a patient, a physician or trained personal; has to penetrate the needle manually. Though, said device comprises a needle shield sleeve for activating the injection step and for covering the needle after the medicament has been injected, some users experience this manually penetration as an unpleasant step.
A solution to this problem is disclosed in document U.S. Pat. No. 5,104,380 which discloses an injection device having a rotatable dose metering cap which compresses a coil spring. When a dose is to be injected, a latch body is pressed against an injection site such that a compression spring can be decompressed for performing an auto-penetration. That action also brings a drive gear out of engagement permitting the coil spring to unwind and thereby rotating a drive plunger rod. Said rotational movement is accompanied by axial movement to cause medicament to be discharged from a cartridge and injected through a needle. This design cannot be utilised when larger forces for injecting a high viscous medicament are required. This design also requires an extra latch body comprising an extra spring for auto-penetration, which increases the physical size and manufacturing cost for such device.